1. Field of the Invention
The present invention relates to a toner for use in a developer for developing latent electrostatic images in, for example, electrophotography, electrostatic recording or electrostatic printing and to an electrophotographic developing system using the toner. More specifically, it relates to a toner for electrophotography, electrophotographic developer and electrophotographic development system, which are used, for example, for copiers, laser printers, facsimiles for plain paper using a direct or indirect electrophotographic developing system. Further, the present invention relates to a toner for electrophotography, an image forming apparatus (development system) and a process cartridge, which are used for full color copiers, full color laser printers, and full color plain paper facsimile machines using a direct or indirect electrophotographic multicolor developing system.
2. Description of the Related Art
In electrophotography, electrostatic recording, and electrostatic printing, a developer is, for example, applied to a latent electrostatic image bearing member such as a photoconductor, so as to dispose the developer onto a latent electrostatic image formed on the latent electrostatic image bearing member in a developing step, the developer disposed on the image is transferred to a recording medium such as a recording paper in a transferring step, thereafter the transferred developer is fixed on the recording medium in a fixing step. Such developers used for developing the latent electrostatic image formed on the latent electrostatic image bearing member generally include two-component developers containing a carrier and a toner, and one-component developers such as magnetic toners and non-magnetic toners, which do not require a carrier. Conventional dry toners for use in electrophotography, electrostatic recording or electrostatic printing are formed by melting and kneading a toner binder (binder resin) such as a styrenic resin or a polyester, a colorant, and other components, then pulverizing the kneaded substance.
These dry toners are, after used for developing and transferred on a recording medium such as a sheet of paper, fixed on the sheet by heating and melting the toner using a heat roller. If a temperature of the heat roller is excessively high, in this procedure, “hot offset” occurs. Hot offset is the problem that the toner is excessively melted and adhered onto the heat roller. If a temperature of the heat roller is excessively low, on the other hand, a degree of melting the toner is insufficient, resulted in insufficient image fixing. Accordingly, there are demands in a toner having a higher temperature at which hot offset occurs (excellent hot offset resistance) and a low fixing temperature (excellent image-fixing properties at low temperatures), in view of energy conservation and miniaturization of apparatuses such as copiers. Toners also require a heat-resistant storability that suppresses blocking of toner when the toner is stored, and at a temperature of atmosphere inside the apparatus where the toner is accommodated. Especially, low melting viscosity of toner is essential in full-color copiers and full-color printers in order to yield high gloss and excellent color mixture of an image. As a consequence, a polyester toner binder which melts sharply has been used in such a toner. However, this toner tends to cause hot offset. To prevent hot offset, in full-color apparatuses, silicone oil has conventionally been applied on the heat roller. Yet, in the method of applying silicone oil to the heat roller, the apparatuses need to equip an oil tank and an oil applier, therefore the apparatuses become more complex in their structures and large in their size. It also leads to a deterioration of the heat roller, so maintenance is required at every certain term. Further, it is unavoidable that the oil is attached to recording media such as copier paper and films for OHP (over head projector), and especially with the films for OHP, the attached oil causes deterioration in color tone.
To prevent a toner fusion without applying oil to a heat roller, wax is generally added to a toner. In this method, however, releasing effect is largely affected by a condition of dispersed wax within a toner binder. Wax does not exhibit its releasing ability if the wax is compatible with a toner binder. Wax exhibits its releasing ability and improves releasing ability of toner when the wax stays within a toner binder as incompatible domain particles. If a diameter of domain particles is excessively large, the resulting toner may not yield images with good quality. This is because a ratio of wax occurring in a surface portion of a toner with respect to other components of the toner increases with an increasing diameter thereof. As a result, the toner particles aggregate to impair fluidity of the toner. Moreover, filming occurs where the wax migrates to a carrier or a photoconductor during long-term use. Color reproducibility and clearness of an image are impaired in the case of color toners. On the contrary, if a diameter of the domain particles is excessively small, the wax is excessively finely dispersed so that sufficient releasing ability cannot be obtained. Although it is necessary to control a diameter of wax as mentioned above, an appropriate method thereof has not been found yet. For example, in the case of toners manufactured by pulverization, control of wax diameter largely relies upon shear force of mixing during melting and kneading procedures. Polyester resins recently used for a toner binder have a low viscosity, and sufficient shear force cannot be added thereto. It is very difficult to control distribution of wax and to obtain a suitable diameter especially for these toners.
Another problem of pulverization is that wax is likely to be exposed at a surface of toner, since a toner material article (for example a toner block) tends to break at a plane where the wax occur as a result of pulverization, and such planes constitute the surface of the toner particles.
Although improvement of toners has been attempted by miniaturizing a diameter of toner particle or narrowing particle diameter distribution of toner in order to obtain high quality images, uniform particle shape cannot be obtained by ordinary manufacturing methods of kneading and pulverization. Moreover, the toner is further pulverized so that excessively fine toner particles are generated, in a course of mixing with carrier in a developing member of the apparatus, or, by a contact stress between a development roller, and a toner applying roller, a layer thickness controlling blade, or a friction charging blade. These lead to deterioration of image quality. In addition, a fluidizer embedded in the surface of toner also leads to deterioration of image quality. Further, fluidity of the toner particles is insufficient because of their shapes, and thus a large amount of the fluidizer is required or a packing fraction of the toner into a toner vessel becomes low. These factors inhibit miniaturization of apparatuses.
A process for transferring, in which an image formed by a multicolor toner is transferred to a recording medium or a sheet of paper, becomes more and more complicated in order to form full-color images. When toners having non-uniform particle shapes, and therefore insufficient transferring ability, such as pulverized toners are used in such a complicated transferring process, missing portions can be found in the transferred image or an amount of the toner consumption becomes large to compensate for the low transferability of the toner.
Accordingly, a strong demand has arisen to yield high quality images which do not have any missing part and to reduce running cost by further improving transfer efficiency leading to a reduction in toner consumption. If transfer efficiency is remarkably excellent, a cleaning unit, which removes remained toner on a photoconductor or a transfer after transferring, can be omitted from an apparatus. Therefore, the apparatus can be miniaturized and low cost thereof can be achieved together with having a merit of reducing a waste toner. Hence, various methods for manufacturing a spherical toner have been suggested in order to overcome the defects caused by a non-uniformly shaped toner.
Various investigations have been done to improve properties of toner. For example, a releasing agent (wax) having a low melting point, such as a polyolefin, is added to a toner in order to improve image-fixing properties at low temperatures and offset resistance. JP-A Nos. 06-295093, 07-84401, and 09-258471 disclose toners that contain a wax having a specific endothermic peak determined by DSC (differential scanning calorimetry). However, the toners disclosed in the above patent publications still need to improve image-fixing properties at low temperatures, offset resistance and also developing properties.
JP-A Nos. 05-341577, 06-123999, 06-230600, and 06-324514 disclose candelilla wax, higher fatty acid wax, higher alcohol wax, vegetable naturally occurring wax (carnauba wax and rice wax), and montan ester wax as a releasing agent of toner. However, the toners disclosed in the above patent publications still need to improve developing properties (charging ability) and durability. If the releasing agent having a low softening point is added to a toner, fluidity of the toner is decreased hence developing properties or transferring ability is also decreased. Moreover, charging ability, durability and storability of the toner may be deteriorated thereby.
JP-A Nos. 11-258934, 11-258935, 04-299357, 04-337737, 06-208244, and 07-281478 disclose toners which contain two or more releasing agents in order to enlarge a fixing region (non offset region). However, the releasing agents are not dispersed sufficiently uniformly in these toners.
JP-A No. 08-166686 discloses a toner which contains polyester resin and two types of offset inhibitors having different acid values and softening points. However, the toner is still insufficient in developing properties. JP-A Nos. 8-328293, and 10-161335 each disclose a toner that specifies a dispersion diameter of wax within the toner particle. However, the resulting toner may not exhibit sufficient releasing ability during fixing since a condition or positioning of the dispersed wax is not defined in the toner particle.
JP-A No. 2001-305782 discloses a toner in which spherical wax particles are fixed onto the surface of toner. However, the wax particles positioned on the surface of toner decreases fluidity of the toner and thus developing properties or transferring ability of the toner is also decreased. In addition, charging ability, durability, and storability of the toner may also be adversely affected. JP-A No. 2001-26541 discloses a toner in which wax is included in the toner particle and the wax is located in a surface portion of the toner particle. However, the toner may be insufficient in all of offset resistance, storability, and durability.
Japanese Patent Application Publications (JP-B) No. 52-3304 and No. 07-82255 disclose that in a pulverized toner using a styrenic resin as a toner binder, a polyolefin releasing agent such as a lower molecular weight polyethylene or lower molecular weight polypropylene, or a graft resin comprising such a polyolefin resin grafted with a styrenic resin. However, the styrene resin used herein has insufficient low-temperature image-fixing properties, and the toner is not suitable for energy saving requirements. As a possible solution to this problem, JP-A No. 2000-75549 proposes a combination use with a polyester resin having excellent low-temperature image-fixing properties. However, the toner proposed is finely pulverized toner prepared by kneading and pulverization, in which the material is fused, kneaded, finely pulverized and classified. The toner thereby has an irregular shape and an irregular surface, and its shape and surface configuration cannot be significantly controlled arbitrarily, while these conditions slightly depend on the crushability of the material or conditions in the pulverization process. In addition, the classification ability at present cannot yield a sharper particle distribution of a toner, and such a sharper particle distribution leads to increased cost. In addition, it is difficult for a conventional pulverized toner to have a small average particle diameter of about 6 μm or less in view of the yield, productivity, and cost of production.
JP-A No. 11-133665 proposes a dry toner containing an elongation product of a urethane-modified polyester and having a practical sphericity of 0.90 to 1.00 as a toner binder in order to improve the fluidity, image-fixing properties at low temperatures, and hot offset resistance of the toner. JP-A No. 11-149180 and JP-A No. 2000-292981 disclose dry toners, and production methods thereof, having a small average particle diameter, which are excellent in fluidity, transfer ability, storage stability at high-temperatures, image-fixing properties at low temperatures, and hot offset resistance. These toners may produce glossy images without requiring application of oil to a heat roller, when used in full color copier. In the publications, these toners are prepared by a method including a process for increasing molecular weight in which a polyester prepolymer having an isocyanate group is subjected to additional polymerization with an amine in an aqueous medium. The technique disclosed in JP-A No. 11-133665 may lead to novel features and advantages by employing a urethane reaction to form a binder in the toner but it is still a pulverization process and does not consider to produce a toner having a small particle diameter and a spherical shape. The toners disclosed in JP-A No. 11-149180 and JP-A No. 2000-292981 are prepared by granulation in water. However, in such granulation in water, a pigment in an oil phase aggregates at the interface with an aqueous phase, and the toner has insufficient fundamental properties such as decreased volume resistivity or heterogenous pigment distribution. To produce a small average particle diameter and a satisfactorily controlled shape of a toner for use in a machine without application of oil, the shape and properties of the toner must be precisely controlled. However, the publications fail to teach the control of the shape and properties of the toner, and intended advantages may not be significantly exhibited. In the toner particles prepared by granulation in water, the pigment and wax often gather in the surface of the particles. In addition, toner particles having an average particle diameter of about 6 μm or less have a large specific surface area. To produce desired charging properties and image-fixing properties, design of the particle surface becomes important in addition to the entire design of the polymer component.